A. Field of the Invention
The present invention relates to an air cleaning apparatus for cleaning air in ambient areas in or around processing units of various processing apparatuses.
B. Description of the Related Art
In order to finish processing of an object to a predetermined state without causing any impurities to adhere to the object, an ambient area in or around a processing unit for processing the object must be cleaned. For example, an air cleaner is incorporated in a CVD apparatus for forming a thin film on the surface of semiconductor wafers by chemical vapor deposition, in a semiconductor device manufacturing process. The air cleaner is designed so that impurities in an ambient area of the CVD apparatus are removed by means of a filter in a manner such that air in the apparatus, in which the semiconductor wafers are transported and processed, is circulated by means of a blast fan. In this arrangement, the impurities are prevented from adhering the to the semiconductor wafers, so that the wafers can be filmed with a high degree of accuracy. The following is a detailed description of a case in which the air cleaner of this type is applied to a CVD apparatus.
In general, a CVD apparatus, especially a vertical CVD apparatus, comprises a cylindrical processing unit in which semiconductor wafers are subjected to a filming process and other processes, a wafer boat for holding the wafers in the processing unit, and a transportation mechanism for supporting the wafer boat and delivering it into and from the processing unit. Further, the CVD apparatus comprises a transfer mechanism for transferring the semiconductor wafers to the wafer boat supported by the transportation mechanism, and a housing having a space in which these components of the apparatus are arranged.
A blast fan and a dusting filter for removing dust, such as particles, are arranged in the housing to prevent particles from adhering to the semiconductor wafers which are moved in the internal space of the housing by means of the transfer mechanism or together with the wafer boat. The blast fan forms an air flow which is directed from the rear portion of the space in the housing toward the front portion thereof. By disposing the dusting filter in the air flow in the housing, the air flow can be cleaned at all times, so that the particles can be prevented from adhering to the semiconductor wafers.
Semiconductor wafers have recently been increased in diameter, and their microfine working has been promoted, so that film layers, such as silicon oxide film, silicon nitride film, etc., formed on the wafers have been becoming thinner and thinner. Accordingly, the properties, such as electrical properties, of the film layers require stricter control, and a very small amount of impurities in the processing ambient have come to exert a great influence upon the workmanship of the film layers. In fine working of 16DRAM or more, fine particles which cannot be attributed only to adhesion of dust appear on the surface of the film layers. These fine particles worsen the electrical properties and other properties of the film layers, thereby lowering the yield.
In order to solve this problem, the inventor hereof made a detailed analysis and examination of the ingredients of impurities which would lower the electrical properties or other properties of the film layers of the semiconductor wafers, especially a very small amount of impurity gases in the processing ambient, by freely utilizing the up-to-date instrumental analysis technology. Thereupon, the inventor hereof ascertained that the very small amount of impurity gases constitute a main cause of the production of the aforesaid fine particles. Further, a qualitative analysis of the impurity gases contained in the air in the housing of the CVD apparatus revealed the existence of organic compounds such as hydrocarbon, phosphorus compounds, boron compounds, etc.
Accordingly, the inventor hereof further investigated the source of these impurity gases, and estimated that the main source was the synthetic resin material of the dusting filter, and that a very small amount of compounds remaining in the synthetic resin material had diffused into the housing and formed the impurity gases. Comparison between the compounds remaining in the synthetic resin material and the ingredients of the impurity gases detected by the gas analysis indicated their coincidence. It was also found that the impurity gases are generated from a clean room in which the CVD apparatus is installed, as well as from the dusting filter in the housing. Furthermore, it was indicated that substances which tend to accelerate the diffusion of boron compounds and the like from the dusting filter flow from the clean room into the CVD apparatus. Thus, the very small amount of impurity gases diffusing from the dusting filter (this diffusion is accelerated by the substances flowing from the clean room into the CVD apparatus in some cases), are transported to the processing ambient by means of the blast fan in the housing, and adhere to the film layers on the in-process semiconductor wafers, thereby producing fine particles on the surface of the film layers. In carrying the wafer boat out from the processing unit by means of the transportation mechanism after the filming process, in particular, the temperature in the housing is considerably increased by radiant heat from the wafer boat, so that the temperature of the dusting filter also increases, thereby accelerating the diffusion of the impurity gases from the dusting filter. Immediately after the filming process, moreover, the film layers are heated and chemically active. Due to these two factors, the fine particles are liable to be produced on the surface of the film layers.